The present invention relates to a liquid crystal display device, and in particular to a low-voltage high-speed liquid crystal display device obtained by improving flatness of various thin films of an active matrix substrate fabricated using a polycrystalline silicon semiconductor formed by a laser annealing technique and a fabrication method thereof.
Liquid crystal display devices are widely used as display monitors in information processing terminals and video display devices of TV receivers. The liquid crystal display devices have a basic structure formed of a pair of insulating substrates and a liquid crystal layer contained therebetween, and display pictures or videos by changing orientations of liquid crystal molecules of the liquid crystal layer.
Various types of liquid crystal display devices are known which differ in a method of forming pixels. Among others, an active matrix type is widely adopted which disposes a switching element (an active element) at each pixel on an inner surface of one of a pair of insulating substrates and forms a display image by selecting some of the switching elements.
The most popular one of the active matrix type liquid crystal display devices is a thin film transistor (TFT) type liquid crystal display device which uses thin film transistors as the switching elements.
Recently, polycrystalline silicon semiconductor has been put to practical use as semiconductor layers constituting circuit elements such as thin film transistors and passive circuit components of the thin film transistor type liquid crystal display devices.
FIG. 8 is a schematic plan view of an active matrix substrate for explaining an example of a liquid crystal display device which uses polycrystalline silicon semiconductors. Reference character SUB1 denotes a first substrate (a lower substrate, an active matrix substrate), and scanning signal lines (gate lines) GL and video signal lines (drain lines) DL are arranged vertically and horizontally, respectively, in a display area AR. A thin film transistor TFT is disposed at each intersection of the scanning signal lines GL and the video signal lines DL, and a pixel electrode PT driven by one of the thin film transistors TFT forms a unit pixel.
Fabricated at the periphery of the display area AR on the substrate SUB1 are a vertical scanning drive circuit (a gate drive circuit) V for applying a scanning voltage to the gate lines GL, a horizontal scanning drive circuit (a drain drive circuit) H, and a precharge circuit PG.
Disposed at one side of the SUB1 is a terminal TM for receiving display signals from external equipment (a signal source such as a host computer or video signal processing equipment). Reference character COM denotes a terminal for applying a drive signal to a common electrode formed on the other insulating substrate (not shown).
To fabricate a polycrystalline silicon semiconductor film on an insulating substrate made of glass or quartz (hereinafter referred to merely as a substrate), a method has been generally used which forms an amorphous silicon film on the substrate using a technique such as CVD, then irradiates a laser beam onto the amorphous silicon film to melt locally the amorphous silicon film only and convert it into a polycrystalline silicon film at a temperature at which a low heat-resistance substrate such as a glass substrate is not melted or broken.
This method makes it possible to use relatively inexpensive glass as substrates, and thereby to reduce the cost of liquid crystal display devices and place high-quality liquid crystal display devices on the market.
The method is disclosed in Japanese Patent Application Laid-open No. Hei 10-41234 (laid-open on Feb. 13, 1998), for example, which forms an amorphous silicon film on a substrate using a technique such as CVD, then irradiates a laser beam onto the amorphous silicon film to form a polycrystalline silicon semiconductor film on a low heat-resistance substrate such as a glass substrate
The prior art disclosed by Japanese Patent Application Laid-open No. Hei 10-41234 forms a polycrystalline silicon film by irradiating a laser beam onto a single-layer amorphous silicon film only, but it does not teach a method which forms the second layer made of an amorphous silicon on the first layer made of a polycrystalline silicon film, and then grows crystals from the second layer made of the amorphous silicon film with the first layer made of the polycrystalline silicon film used as nucleuses by irradiating the laser beam onto the second layer of the amorphous silicon film.
Japanese Patent Application Laid-open No. Hei 11-40501 (laid-open Feb. 12, 1999) discloses a prior art which first forms a polycrystalline silicon film by irradiating a laser beam onto the first layer made of an amorphous silicon film, then forms the second layer made of an amorphous silicon film on the first layer made of the polycrystalline silicon film, and then converts the second layer made of the amorphous silicon film into a polycrystalline silicon film by irradiating a laser beam onto the second layer of the amorphous silicon film.
But, in the technique of Japanese Patent Application Laid-open No. Hei 10-41234, there was not a concept of removing impurities from the first layer made of the polycrystalline silicon film; therefore, regions having large concentrations of impurities are present at an interface between the first and second layers made of the polycrystalline silicon films and the impurities hinder the polycrystalline silicon films of the first and second layers from melting together. Consequently, this made it difficult to obtain an integral polycrystalline silicon film having good crystal quality and free from boundaries between the first and second layers made of the polycrystalline silicon films.
The above-mentioned impurities are intended to mean the composition of air, dust particles floating in air, but not impurities intentionally introduced into the polycrystalline silicon film to determine the conductivity type of the polycrystalline silicon film, such as boron, phosphorus or arsenic.
Japanese Patent Application Laid-open No. Hei 7-99321 (laid-open on Apr. 11, 1995) discloses a technique which first forms the first layer made of a polycrystalline silicon film, then stacks the second layer made of an amorphous silicon film on the first layer of the polycrystalline silicon film without exposing the polycrystalline film to the atmosphere, and then converts the second layer of the amorphous silicon film into a polycrystalline silicon film by irradiating a laser beam onto the amorphous silicon film.
But, in the technique of Japanese Patent Application Laid-open No. Hei 7-99321, there is no teaching therein of a concept of planarizing a surface of a polycrystalline silicon film, and therefore the technique did not include a cleaning process for removing protrusions produced in the first layer of the polycrystalline silicon film by irradiation of the laser beam before stacking the second layer of the amorphous silicon film on the first layer of the polycrystalline silicon film. Consequently, in the technique of Japanese Patent Application Laid-open No. Hei 7-99321, it was difficult to obtain a polycrystalline silicon film having a very flat surface, unlike the present invention.